Exhaust-gas cleaning unit with particle filter and nitrogen oxygen store, and operating method therefor

ABSTRACT

An exhaust-gas cleaning unit includes a particle filter and a nitrogen oxide store disposed upstream of the particle filter. A method for operating the exhaust-gas cleaning unit includes a nitrogen oxide regeneration phase and a sulphur regeneration phase for the nitrogen oxide store and a soot regeneration phase for the particle filter. A longer period is selected for the sulphur regeneration phase than for the nitrogen oxide regeneration phase. At least part of the sulphur regeneration phase and at least part of the soot regeneration phase are performed as a combined regeneration phase, in which the regeneration phases are performed in immediate succession or one regeneration phase is performed intermittently at a number of intervals during the other regeneration phase.

FIELD OF THE INVENTION

[0001] The invention relates to an exhaust-gas cleaning unit having aparticle filter and a nitrogen oxide store and to a method for operatinga unit of this type. Exhaust-gas cleaning units of this type aresuitable in particular for cleaning the exhaust gases of diesel enginesfrom, for example, motor vehicles.

BACKGROUND INFORMATION

[0002] European Published Patent Application No. 0 758 713 describes anexhaust-gas cleaning unit in which an oxidation catalytic converter isconnected upstream of the particle filter, as seen in the direction offlow of the exhaust gas, and the nitrogen oxide store is connecteddownstream of the particle filter. As an alternative to the nitrogenoxide store, a nitrogen oxide reduction catalytic converter may be used.The oxidation catalytic converter is used to convert nitrogen monoxidewhich is contained in the exhaust gas in soot regeneration phases of theparticle filter into nitrogen dioxide, which promotes the soot burn-offin the particle filter. The nitrogen oxide store or the nitrogen oxidereduction catalytic converter is used to collect or convert nitrogenmonoxide which is formed by the reaction of the nitrogen dioxide withthe soot particles. The soot regeneration operations occur, for example,every 60 minutes for about three minutes with a lean exhaust-gascomposition and temperatures of between approximately 400° C. and 500°C.Conventional nitrogen oxide regeneration phases for the nitrogen oxidestore are performed approximately every 10 seconds to every few minutes,in each case for approximately 0.5 seconds, with a rich exhaust-gascomposition. When a nitrogen oxide regeneration phase of this type fallsin the period of a soot regeneration phase, it is performed, by anadditional increase in temperature, as a correspondingly short sulphurregeneration phase, in order to prevent gradual sulphur poisoning of thenitrogen oxide store.

[0003] Various methods for nitrogen oxide (NO_(x)) regeneration andsulphur regeneration of a nitrogen oxide store are conventional. Forexample, German Published Patent Application No. 197 50 226 describes amethod for NO_(x) regeneration in which a desired, rich exhaust-gascomposition is generated substantially only by engine measures,including suitable control of exhaust-gas recycling.

[0004] It is an object of the present invention to provide a new type ofexhaust-gas cleaning unit and a corresponding operating method for thisunit.

SUMMARY

[0005] The above and other beneficial objects of the present inventionare achieved by providing an exhaust-gas cleaning unit and method asdescribed herein.

[0006] The exhaust-gas cleaning unit according to the present inventionincludes a nitrogen oxide store connected upstream of the particlefilter. Connecting the nitrogen oxide store upstream of the particlefilter, as seen in the direction of flow of the exhaust gas, has anumber of particular advantages. For example, nitrogen oxides, whichhave been temporarily stored in the nitrogen oxide store, may beliberated at the elevated exhaust-gas temperature which is required forsoot regeneration of the particle filter and may additionally assist thesoot regeneration in the form of nitrogen dioxide (NO₂). Furthermore,sulphur regeneration of the nitrogen oxide store, which requiresrelatively high exhaust-gas temperatures of typically between 600° C.and 700° C., may be combined with soot regeneration of the particlefilter, for which elevated exhaust-gas temperatures of betweenapproximately 400° C. and approximately 600° C. are generally used, thefact that the nitrogen oxide store is connected upstream of the particlefilter resulting in a natural temperature gradient along the exhaust-gasflow which matches these temperature requirements.

[0007] A respective oxidation catalytic converter may be providedupstream of the nitrogen oxide store, between the nitrogen oxide storeand the particle filter and/or downstream of the particle filter.Depending on the particular arrangement, an oxidation catalyticconverter of this type assists with raising the exhaust-gas temperature,soot regeneration by generating NO₂ and/or avoiding hydrocarbon (HC)and/or carbon monoxide (CO) emissions.

[0008] An HC/CO/O₂ storage coating and/or an oxidation catalyst coatingand/or a coating which promotes the oxidation of soot may be provided inthe particle filter, preferably in each case in an inlet-side partthereof. Thus, unburned hydrocarbons and carbon monoxide may be trappedor oxidized and/or the soot burn-off during the particle filterregeneration may be assisted.

[0009] An exhaust-gas according to the present invention may include alambda probe downstream of the particle filter. This probe may be usedto monitor the soot burn-off behavior during the particle regenerationand to detect any breakthrough of reducing agents during the NO_(x)regeneration of the nitrogen oxide store.

[0010] The operating method according to the present invention includesfirstly sulphur regeneration phases for the nitrogen oxide store, whichare performed for a longer period than the NO_(x) regeneration phases,in order to achieve complete sulphur desorption. Moreover, combinedsulphur and soot regeneration phases are provided, during which thesulphur regeneration of the nitrogen oxide store and the sootregeneration of the particle filter may be linked in terms of time bythe two regeneration operations being performed immediately after oneanother or by one regeneration operation being performed intermittentlyat intervals during the other regeneration operation. In this manner,the thermal exhaust-gas energy may be utilized for both regenerationprocesses.

[0011] In the operating method according to the present invention, alambda probe downstream of the particle filter may be used firstly todetect the progress of a soot regeneration phase and secondly to detecta breakthrough of reducing agents during a nitrogen oxide regenerationphase and therefore that the latter phase has ended.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a schematic block diagram of an exhaust-gas cleaningunit for a diesel engine having a particle filter, an upstream nitrogenoxide store and an optional downstream oxidation catalytic converter.

[0013]FIG. 2 is a schematic block diagram of an exhaust-gas cleaningunit having an oxidation catalytic converter disposed upstream of thenitrogen oxide store.

[0014]FIG. 3 is a schematic block diagram of an exhaust-gas cleaningunit having an oxidation catalytic converter disposed between thenitrogen oxide store and the particle filter.

[0015]FIG. 4 is a schematic block diagram of an exhaust-gas cleaningunit having a particle filter that includes a coating configured toperform an exhaust-gas cleaning function.

DETAILED DESCRIPTION

[0016] The exhaust-gas cleaning unit, only the relevant components ofwhich are illustrated in FIG. 1, is suitable in particular for cleaningthe exhaust gas from a diesel engine, for example in a motor vehicle.The exhaust-gas cleaning unit includes, as components which are activein the exhaust-gas cleaning in an exhaust system 1, a particle filter 2and a nitrogen oxide store 3 which is connected upstream of the particlefilter 2, as seen in the direction R of flow of the exhaust gas, as wellas, optionally, an oxidation catalytic converter 4 connected downstreamof the particle filter 2.

[0017] A control unit, which may formed, for example, by an enginecontrol unit which controls the exhaust-emitting diesel engine, is usedto control the operation of the exhaust-gas cleaning unit. The controlunit determines the operating state of the exhaust-gas cleaning unitusing various sensors arranged in the exhaust system 1. These sensorsinclude in particular a first sensor arrangement S1. Upstream of thenitrogen oxide store 3, for detecting the lambda value, the nitrogenoxide content and the temperature of the exhaust gas, a second and thirdsensor arrangement S2, S3 between the nitrogen oxide store 3 and theparticle filter 2 and downstream of the latter, respectively, in eachcase to determine pressure and temperature, and, also downstream of theparticle filter 2, upstream or downstream of the optional oxidationcatalytic converter 4, a fourth sensor arrangement S4 for determiningthe lambda value and oxygen and/or nitrogen oxide content. Depending onthe particular application, only some of the abovementioned sensors maybe provided.

[0018] FIGS. 2 to 4 illustrate variants of the unit illustrated inFIG. 1. In the unit illustrated in FIG. 2, an oxidation catalyticconverter 5 is additionally provided upstream of the nitrogen oxidestore 3. In this case, the optional oxidation catalytic converter 4downstream of the particle filter 2 may be smaller.

[0019] In the exhaust-gas cleaning unit illustrated in FIG. 3, anoxidation catalytic converter 6 is connected between the nitrogen oxidestore 3 and the particle filter 2. Again, the optional oxidationcatalytic converter 4 downstream of the particle filter 2 may besmaller.

[0020] The exhaust-gas cleaning unit illustrated in FIG. 4 uses amodified particle filter 2 a which, in an inlet-side section 7, isprovided with a coating which is active in cleaning of the exhaust gas.Depending on the particular application, this coating is selected sothat it fulfils an oxidation catalyst function or an HC/CO/O₂ storagefunction or a function of promoting soot burn-off. Suitable materialsfor such coatings are conventional and therefore require no furtherexplanation. In the first case, the coating acts as an oxidationcatalyst, i.e., it catalyzes oxidation of gaseous, oxidizableexhaust-gas constituents. In the second case, the coating serves tostore unburned hydrocarbons, carbon monoxide or oxygen contained in theexhaust gas at the inlet side of the particle filter 2 a, depending onthe operating state of the diesel engine and of the exhaust-gas cleaningunit. In this manner, it is possible, for example, to prevent abreakthrough of unburned hydrocarbons and of carbon monoxide. In thethird case, the coating serves as an oxidation aid in the soot burn-offduring the soot regeneration of the particle filter, with the resultthat the soot burn-off may occur even at relatively low temperatures.

[0021] Because the nitrogen oxide store 3 is arranged upstream of theparticle filter 2, each of the installations illustrated may be operatedso that the functions of these two exhaust-gas cleaning components 2, 3may be adapted to one another and assist one another with optimumutilization of the thermal energy contained in the exhaust gas. As isconventional, the nitrogen oxide store 3 fulfills the nitrogen oxidereduction function, by temporarily storing nitrogen oxides, for exampleby adsorption in nitrate form, during lean operation of the dieselengine and releasing them and reducing them to form nitrogen duringperiodic regeneration or desorption phases. These NO_(x) regenerationphases are performed at typical intervals of one to a few minutes, for aperiod of typically slightly less than one second to a few seconds,during which period a rich exhaust-gas composition is set. Any secondaryemissions of unburned hydrocarbons and carbon monoxide which arise maybe oxidized by the subsequent oxidation catalytic converter(s) 4, 6.

[0022] In the unit illustrated in FIG. 4, a breakthrough of unburnedhydrocarbons and/or carbon monoxide may also be prevented as a result ofthe particle filter coating 7 containing a material which has anHC/CO/O₂ storage function and, depending on the exhaust-gas atmosphereand exhaust-gas temperature, is able to temporarily store hydrocarbonsand carbon monoxide and/or to oxidize them using temporarily storedoxygen.

[0023] The particle filter 2 collects particles contained in the exhaustgas for a running distance of typically of the order of magnitude of afew hundred kilometers or several operating hours of the diesel engine,until its storage capacity is reached, after which it is subjected tosoot regeneration. For this soot regeneration, the exhaust gas is raisedto a suitably high temperature of typically 400° C. to 600° C. bycorresponding engine measures, and a lean exhaust-gas composition withan oxygen content of preferably greater than 5% is established. Thisoperation is typically performed for a few minutes, after which periodthe soot has been completely burnt off and the particle filter 2 hasbeen regenerated.

[0024] The soot regeneration of the particle filter 2 may be assisted bythe upstream nitrogen oxide store 3 as a result of the latter previouslybeing laden with nitrogen oxides. Then, as soon as the exhaust-gastemperature for starting a soot regeneration operation is raised to over400° C., e.g., to over 450° C., the nitrogen oxide store 3, under thelean exhaust-gas atmosphere, releases additional NO₂, which acts as asoot burn-off agent and assists soot burn-off in the downstream particlefilter, which begins at particle filter temperatures of below 400° C.,with the result that the regeneration speed of the particle filter 2 isincreased. If the diesel engine includes an exhaust-gas recyclingsystem, the soot regeneration of the particle filter 2 may be enhancedfurther by deactivating the exhaust-gas recycling during this period,leading to higher NO_(x) raw emissions. As a result, additional nitrogendioxide for accelerated soot burn-off is provided to the upstreamoxidation catalytic converter 5 directly and via the oxidation ofnitrogen monoxide to form NO₂ in the NO_(x) store 3.

[0025] If, in the unit illustrated in FIG. 4, the particle filtercoating 7 is formed by a material which assists with soot oxidation, thesoot burn-off is promoted by this coating 7, which has the effect ofcausing the soot to be burnt off even at a relatively low temperature.

[0026] When using sulphur-containing operating substances, in particularfuel and oil, for the diesel engine, the nitrogen oxide store 3 maygradually become covered with sulphur contained in the exhaust gas and,as a result, loses its ability to store NO_(x). In order for the sulphurwhich has been incorporated in the nitrogen oxide store 3, generally insulphate form, to be released again, it is conventional to performcorresponding desulphating phases from time to time. These phases aretypically required in each case after a few thousand operatingkilometers and if possible are maintained for a few minutes, typicallyup to about 15 minutes for complete sulphur regeneration. The sulphurregeneration requires the nitrogen oxide store 3 to be heated to arelatively high temperature of, typically, above 600° C., e.g. to 650°C.

[0027] The times at which a sulphur regeneration of the nitrogen oxidestore 3 is performed may be linked to soot regeneration of the particlefilter 2, since both operations require elevated exhaust-gastemperatures and therefore the elevated thermal exhaust-gas energy maybe utilized for both regeneration operations. By way of example, sulphurregeneration may be performed immediately before or immediately after asoot regeneration. A further possibility is for both regenerationprocesses to be performed quasi-simultaneously, as a result of theprocess parameters for one regeneration operation being established inprinciple for over and above a certain period, but during this periodthe process parameters are intermittently switched over for briefperiods to those required for the other operation. By way of example, itis possible, during soot regeneration of the particle filter 2, duringwhich in principle a lean exhaust-gas composition is established, tointermittently, for example every 15 seconds to 60 seconds, switch overfor a brief period of, for example, three seconds to 10 seconds, to arich exhaust-gas composition, and in this manner to bring about sulphurregeneration of the nitrogen oxide store 3.

[0028] This time-matching of soot regeneration of the particle filter 2and sulphur regeneration of the nitrogen oxide store 3 is assisted bythe specific arrangement of the nitrogen oxide store 3 upstream of theparticle filter 2 and the standard temperature gradient in the exhaustsystem 1, since this arrangement means that, at a given, elevatedexhaust-gas temperature, the nitrogen oxide store 3 tends to be at ahigher temperature than the particle filter 2 which follows itdownstream. In this manner, it is possible, without further additionalheating measures, to set the temperature of, for example, 650° C. whichis required for the sulphur regeneration in the nitrogen oxide store 3and, at the same time, to set the slightly lower temperature ofapproximately 400° C. to 600° C. required for soot regeneration in theparticle filter 2. Moreover, the relatively high thermal inertia of theparticle filter, which results from its mass, does not cause any delayto the temperature control of the NO_(x) store 3. Furthermore, theincrease in the exhaust-gas temperature may be at least partiallyaffected by oxidation, for example, of additionally injected fuel.

[0029] Connecting the oxidation catalytic converter 5 upstream of thenitrogen oxide store 3, in accordance with the unit illustrated in FIG.2, given its particularly high temperature stability, may contribute toprotecting the nitrogen oxide store 3 from very high exothermictemperatures during this oxidation, in that it oxidizes at least some ofthe oxidizable constituents contained in the exhaust gas, and as aresult, at least partially relieves the nitrogen oxide store 3 of thisoxidation function. In this manner, the thermal aging of the nitrogenoxide store 3 may be minimized.

[0030] As the above description of example embodiments illustrates, theexhaust-gas cleaning unit according to the present invention and theassociated operating method according to the present invention allow thethermal exhaust-gas energy to be utilized for the regenerationoperations of both the nitrogen oxide store 3 and the particle filter 2.Moreover, the upstream nitrogen oxide store 3 is able to assist andaccelerate the soot regeneration of the particle filter 2 byadditionally providing nitrogen oxide and by increasing the exhaust-gastemperature by exothermic oxidation of, for example, fuel which isintroduced into the exhaust gas by additional injection into the dieselengine.

[0031] The exhaust-gas sensor arrangements S1 to S4 of the exhaust-gascleaning unit, for measuring the temperature, the pressure and thenitrogen oxide and oxygen content in the exhaust gas at the variouslocations of the exhaust system, may be used to control the sequence ofthe various regeneration operations described above. For example,arranging the lambda probe S4 downstream of the particle filter 2upstream or downstream of the optional oxidation catalytic converter 4allows the exhaust gas to be monitored both with regard to abreakthrough of reducing agent during nitrogen oxide regenerationphases, which indicates that the nitrogen oxide desorption has beencompleted, and with regard to the oxygen content during the sootregeneration of the particle filter 2, with the result that the burn-offbehavior of the soot may be determined and complete conclusion of thesoot burn-off may be detected.

What is claimed is:
 1. An exhaust-gas cleaning unit, comprising: aparticle filter; and a nitrogen oxide store, the nitrogen oxide storebeing disposed upstream of the particle filter.
 2. The exhaust-gascleaning unit according to claim 1, wherein the exhaust-gas cleaningunit is configured for a diesel engine of a motor vehicle.
 3. Theexhaust-gas cleaning unit according to claim 1, further comprising anoxidation catalytic converter, the oxidation catalytic converter beingdisposed at least one of between the nitrogen oxide store and theparticle filter, upstream of the nitrogen oxide store and downstream ofthe particle filter.
 4. The exhaust-gas cleaning unit according to claim1, wherein the particle filter includes a coating configured to performone of an oxidation catalytic converter function, an HC/CO/O₂ storagefunction and a soot oxidation assisting function.
 5. The exhaust-gascleaning unit according to claim 1, further comprising a lambda probedisposed downstream of the particle filter.
 6. A method for operating anexhaust-gas cleaning unit having a particle filter and a nitrogen oxidestore disposed upstream of the particle filter, the method comprisingthe steps of: performing a nitrogen oxide regeneration phase with, atleast temporarily, a rich exhaust-gas composition for the nitrogen oxidestore; performing a sulphur regeneration phase with an elevatedtemperature and, at least temporarily, a rich exhaust-gas compositionfor the nitrogen oxide store; and performing a soot regeneration phasewith, at least temporarily, a lean exhaust-gas composition and anelevated exhaust-gas temperature for the particle filter; wherein alonger period is provided for the sulphur regeneration phase than forthe nitrogen oxide regeneration phase; and wherein at least part of thesulphur regeneration phase and at least part of the soot regenerationphase are performed as a combined sulphur and soot regeneration phase,the combined sulphur and soot regeneration phase including one of: aplurality of shorter intermittent sulphur regeneration phases during alonger soot regeneration phase; a plurality of shorter intermittent sootregeneration phases during a longer sulphur regeneration phase; a sootregeneration phase and a sulphur regeneration phase in immediatesuccession; and a sulphur regeneration phase and a soot regenerationphase in immediate succession.
 7. The method according to claim 6,wherein the exhaust-gas cleaning unit includes a lambda probe disposeddownstream of the particle filter, the method further comprising thesteps of: monitoring the exhaust-gas composition with the lambda probeduring the nitrogen oxide regeneration phase for a breakthrough ofreducing agent, the breakthrough of reducing agent indicating an end ofthe nitrogen oxide regeneration phase; and monitoring the exhaust-gascomposition with the lambda probe during the soot regeneration phase foroxygen content, the oxygen content being indicative of an end of a sootburn-off.